1. Field of the Invention
The invention relates to image processing method and apparatus and image forming method and apparatus using such method and apparatus.
2. Related Background Art
Among conventional image forming apparatuses, for example, there is an apparatus such that each pixel is expressed as a density by multi-values in a dividing image of predetermined pixels, what is called a digital image, by an electrophotography process. Exposure amounts of all pixels are modulated in accordance with an image signal value of each pixel, thereby reconstructing image information at a high fidelity. When the exposure amounts of all pixels are modulated, however, there is a problem such that a density gradation of an output image becomes nonlinear (generally, not enough density is obtained in a highlight density region and a gamma value is large in a halftone density region) for an input image signal, so that a picture quality deteriorates.
As another example of the image forming apparatus, there is an apparatus such that in the case of a pixel composition of an input image of a pixel density of, for example, 400 d.p.i., an output image is set to 200 d.p.i. in the main-scanning direction and to 400 l.p.i. in the sub-scanning direction (direction perpendicular to the main scanning direction), and the pixel density in the main-scanning direction (direction for light scanning) is set to the half density and a density gradation of the output image is made to linearly correspond to the input image, thereby forming an image.
In the above conventional image forming apparatus, however, there are the following problems. Namely, when the pixel density in the main-scanning direction of the output image is reduced to 1/2 of the pixel density in the sub-scanning direction, the whole resolution of the image is reduced to 1/2 and sharpness of the image deteriorates. In case of outputting image data of a high spatial frequency such as a mesh image or the like, an interference with a recording pixel density easily occurs, and a moire strongly appears.
To solve the above problems, in U.S. Pat. No. 5,148,287, U.S. Pat. No. 5,130,808, and the like, there is disclosed a technique such that when a density level is low, by concentrating the dots, a density reproducibility in a highlight density region is improved.
However, according to the method disclosed in the above two patents, there are problems such that what is called a density jump occurs at a boundary between the density region where the dots are concentrated and the density region where the dots are not concentrated. Such a density jump is observed as a pseudo outline by the human eyes, and the picture quality deteriorates.